Fast-switching electro-optic modulators and method of making the same

Abstract

An electro-optic modulator includes a doped semiconductor crystal having a crystallographic surface having an amplitude modulation orientation, a first metal electrode located on a first surface of the doped semiconductor crystal, a second metal electrode located on a second surface of the doped semiconductor crystal, and accumulation space charge regions located within surface regions of the doped semiconductor crystal that are proximal to the first metal electrode and the second metal electrode and including excess charge carriers of a same type as majority charge carriers of the doped semiconductor crystal.

The invention claimed is: 
     
       1. An electro-optic modulator comprising:
 a doped semiconductor crystal having a crystallographic surface having an amplitude modulation orientation; 
 a first metal electrode located on a first surface of the doped semiconductor crystal; 
 a second metal electrode located on a second surface of the doped semiconductor crystal; and 
 accumulation space charge regions located within surface regions of the doped semiconductor crystal that are proximal to the first metal electrode and the second metal electrode and including excess charge carriers of a same type as majority charge carriers of the doped semiconductor crystal. 
 
     
     
       2. The electro-optic modulator of  claim 1 , wherein:
 the doped semiconductor crystal has a shape of a bar of a uniform cross-sectional shape within cross-sectional planes that are perpendicular to a lengthwise direction; 
 a length of the bar is in a range from 25 mm to 100 mm; and 
 the uniform cross-sectional shape has an area in a range from 6.25 mm 2  to 100 mm 2 . 
 
     
     
       3. The electro-optic modulator of  claim 2 , wherein the first surface and the second surface of the doped semiconductor crystal are two (1Ī0) surfaces of the bar. 
     
     
       4. The electro-optic modulator of  claim 2 , wherein the first surface and the second surface of the doped semiconductor crystal are two (111) surfaces of the bar. 
     
     
       5. The electro-optic modulator of  claim 2 , wherein:
 the bar has a shape of a rectangular prism; and 
 at least two side surfaces of the bar that are perpendicular to the first and second surfaces have an optical surface finish. 
 
     
     
       6. The electro-optic modulator of  claim 1 , wherein the doped semiconductor crystal comprises Cd 1-x Zn x Te in which x have a value in a range from, and including, 0.0 to, and including, 0.15. 
     
     
       7. The electro-optic modulator of  claim 1 , wherein the doped semiconductor crystal has electrical resistivity in the range from 1.0×10 8  Ω·cm to 1.0×10 11  Ω·cm. 
     
     
       8. The electro-optic modulator of  claim 1 , wherein:
 the doped semiconductor crystal has an n-type doping; and 
 the first and second metal electrodes comprise a respective metal having a respective work function that is lower than electron affinity of the doped semiconductor crystal. 
 
     
     
       9. The electro-optic modulator of  claim 8 , wherein each of the first and second metal electrodes comprises a metal selected from indium, aluminum, and silver. 
     
     
       10. The electro-optic modulator of  claim 1 , wherein:
 the doped semiconductor crystal has a p-type doping; and 
 the first and second metal electrodes comprise a respective metal having a respective work function that is higher than electron affinity of the doped semiconductor crystal. 
 
     
     
       11. The electro-optic modulator of  claim 10 , wherein each of the first and second metal electrodes comprises a metal selected from gold, platinum, and nickel. 
     
     
       12. The electro-optic modulator of  claim 1 , further comprising at least one interfacial oxide layer located between the doped semiconductor crystal and one of the first and second metal electrodes and providing reduction in Schottky barrier height, wherein the at least one interfacial oxide layer comprises an oxide of cadmium telluride or an oxide of cadmium zinc telluride. 
     
     
       13. The electro-optic modulator of  claim 1 , further comprising a resistor in a parallel connection with the doped semiconductor crystal and providing a conduction path for surface space charges of the doped semiconductor crystal. 
     
     
       14. The electro-optic modulator of  claim 1 , wherein an extinction coefficient of the doped semiconductor crystal transitions at least by 90% of a difference between an on-value and an off-value within 1 nanosecond of switching of the electro-optic modulator. 
     
     
       15. A method of manufacturing an electro-optic modulator, comprising:
 providing a doped semiconductor crystal having a crystallographic surface having an amplitude modulation orientation; 
 forming a first metal electrode on a first surface of the doped semiconductor crystal; and 
 forming a second metal electrode on a second surface of the doped semiconductor crystal, wherein: 
 accumulation space charge regions are formed within surface regions of the doped semiconductor crystal that are proximal to the first metal electrode and the second metal electrode; and 
 the accumulation space charge regions include excess charge carriers of a same type as majority charge carriers of the doped semiconductor crystal. 
 
     
     
       16. The method of  claim 15 , wherein:
 the doped semiconductor crystal has a shape of a bar of a uniform cross-sectional shape within cross-sectional planes that are perpendicular to a lengthwise direction; 
 a length of the bar is in a range from 25 mm to 100 mm; and 
 the uniform cross-sectional shape has an area in a range from 6.25 mm 2  to 100 mm 2 . 
 
     
     
       17. The method of  claim 16 , wherein the first surface and the second surface of the doped semiconductor crystal are two (1Ī0) surfaces of the bar, or are two (111) surfaces of the bar. 
     
     
       18. The method of  claim 16 , wherein:
 the bar has a shape of a rectangular prism; and 
 the method further comprises providing optical surface finish to at least two side surfaces of the bar that are perpendicular to the first and second surfaces. 
 
     
     
       19. The method of  claim 18 , wherein the optical surface finish to at least two side surfaces of the bar is provided by lapping and polishing the at least two side surfaces of the bar employing alumina slurry having an average particle size in a range from 25 nanometers to 100 nanometers. 
     
     
       20. The method of  claim 15 , wherein:
 the doped semiconductor crystal comprises Cd 1-x Zn x Te in which x have a value in a range from, and including, 0.0 to, and including, 0.15; and 
 the doped semiconductor crystal has electrical resistivity in the range from 1.0×10 8  Ω·cm to 1.0×10 11  Ω·cm. 
 
     
     
       21. A method of operating the electro-optic modulator of  claim 1 , comprising switching the electro-optic modulator, wherein an extinction coefficient of the doped semiconductor crystal transitions at least by 90% of a difference between an on-value and an off-value within 1 nanosecond of the switching of the electro-optic modulator.